1. Field of the Invention
The present invention generally relates to a noise reducing apparatus for reducing a noise component included in a digital video signal. More particularly, the present invention relates to reduction of a noise component included in a digital video signal obtained by compressively encoding image data on a block-by-block basis by an encoder and then decoding the resultant image data by a decoder.
2. Description of the Related Art
A digital video signal resulting from decoding of image data by a decoder includes random noise and encoding noise which is introduced in encoding process. The random noise results from degraded quality of a material image itself to be encoded by an encoder. The encoding noise is generated by, e.g., quantization error caused upon encoding. The encoding noise is thus specific to an encoding/decoding system.
Specific examples of the encoding noise include mosquito noise and blocking artifact. For example, the mosquito noise is generated in a flat region near the edge of the image within a 8×8 pixel encoding block. The blocking artifact appears as distortion at the boundary of an encoding block. These noises are generally reduced or reduced by a smoothing process. Since the noise reducing process is a process that damages the original image in a sense, it involves degradation in image quality more or less. Therefore, the noise reduction level in the noise reducing process is generally determined so as to balance the noise reducing effect with the influence of degradation in image quality.
With recent diversification of image information and display devices, the original image data is increasingly scaled for display rather than being displayed in its original size. For example, the display image size on a display device may vary depending on the areas (e.g., the European countries and the US). Even in the same area, the display image size (aspect ratio) may vary like 4:3 and 16:9 depending on the display devices. Some display devices are capable of scaling the displayed image through operation of the user. Moreover, devices capable of recording and reproducing the image of various sizes such as a DVD player/recorder are becoming widespread. Such an image scaling process is conducted by performing a prescribed operation (a kind of filtering process) on the pixel data of each pixel and its neighboring pixels.
Conventionally, however, since the noise reducing process and the scaling process are independent processes having completely different purposes from each other, the balance between the respective influences of the processes has not been considered particularly. This may result in failing to obtain a sufficient noise reducing effect or may result in degraded image quality (other than noise) when the image data is subjected to such a noise reducing process and scaling process. In other words, even if the noise reducing process is conducted so as to balance the noise reducing effect with the influence of degradation in image quality as described above, the balance is not necessarily retained even after the scaling process. For example, if a smoothing process for noise reduction results in less sharp edges, the edges of the image may get much less sharp as a result of the enlarging process. On the contrary, even if the noise reduction level is increased, the subsequent scaling process (a kind of filtering process) may degrade the image quality rather than improving it.
Similar problems regarding the balance between the noise reducing effect and the influence of degradation in image quality are also caused by the difference in magnitude of the motion in the image. More specifically, the conventional noise reducing apparatus has a fixed, preset noise reduction level. Therefore, if the noise reduction level is set so that noise is sufficiently reduced from a fast-moving image (i.e., an image that reduces human eye's visual sensitivity to degradation in image quality), degradation in image quality is more likely to be perceived in a slow-moving image (i.e., an image that increases the human eye's visual sensitivity thereto) than the noise reducing effect. On the other hand, if the noise reduction level is set so that the slow-moving image is not noticeably degraded in quality, the noise reducing effect on the fast-moving image is not likely to be perceived.